In a conventional electrostatic generator, electric charge is accumulated on a high voltage electrode which is insulated from ground. The high voltage electrode is charged by an endless belt or chain which carries charge to the high voltage electrode. If a large negative charge is put on the high voltage electrode, the electric charge on the electrode can be used to accelerate electrons.
Electrostatic generators are capable of producing extremely high voltages ranging up to over twenty-five million volts. Such high voltage generators have been widely used for operating linear accelerating tubes in which protons, electrons, or other charged particles are accelerated to extremely high energies.
One use for high energy electrons is for the production of high energy x-rays which are used in industry for imaging the interior of rocket motor grains and heavy duty equipment. A high energy high brightness x-ray source is created by focusing a high energy electron beam on to a target of high atomic weight material. In creating a high energy source of electrons, a supply of electrons is fed to the acceleration tube which is positioned between the high voltage electrode and ground. A smooth voltage drop is developed along the acceleration tube by a multiplicity of charge carrying plates which are evenly spaced between the high voltage electrode and the ground. A high value resistor between successively spaced electrodes assures an even cascade of voltages so that the accelerating field is substantially constant between the high voltage electrode and the ground.
As electrons move along the linear acceleration tube, they are accelerated to high energies by the static potential field, developing an energy in electron volts equivalent to the static field through which they have been accelerated. The accelerating electrodes are normally spaced apart by insulating spacers bonded therebetween. The interior of the acceleration tube is maintained at a high vacuum to allow the free flow of electrons.
The tube and the entire electrostatic generator is normally contained within a pressure vessel containing high pressure gas, typically sulphur hexaflouride, which resists electrostatic breakdown between the accelerating electrodes and between the high voltage electrode and the ground. Circular rings are attached to the accelerating electrodes external to the acceleration tube. The rings serve as spark arresters which prevent arcing between the acceleration electrodes within the tube The rings typically have protrusions which extend length-wise in the direction of the potential field. The result of the protrusions is that a narrower gap exists between the spark arresting rings than between the accelerating electrodes themselves. The spark gap protrusions are spaced apart such that any arcing which takes place will be between the spark arresting tings rather than internal to the acceleration tube where the arcing could cause permanent damage to the function of the tube.
One problem that has arisen in the design of high voltage electrostatic accelerators for electrons is that the lower mass of the electrons per unit charge (approximately one two-thousandth that of a proton) makes the electrons unusually sensitive to small DC or low frequency magnetic fields. Because the electron tube is normally contained within a steel pressure vessel, a certain amount of magnetic shielding of the accelerator is performed by the metal casing. However, some sources of fluctuating magnetic fields are intemal to the pressure vessel, such as the motors used to drive the static generators. Further, the pressure vessel itself may introduce stray magnetic fields produced by residual magnetic properties of the shield itself.
Electrostatic accelerators are used to produce high energy particle beams in a variety of applications. Because the particles in these accelerators are electrically charged, their trajectories can be altered by magnetic fields through which they travel. Indeed, this property is utilized by many devices for controlling charged particle beams. However, there are many sources of detrimental stray magnetic fields in accelerators such as: electric motors, generators, ferromagnetic or electromagnetic components, steel pressure vessels, or the earth itself. These fields can reroute the charged particle beams in a complex and unpredictable way making transmission of the beam to the desired target difficult or even impossible. They may cause the accelerator to malfunction. They can also reduce the beam quality by defocusing the beam or changing its position in a time-variant manner. These effects are deleterious to successful operation.
The consequences of the beam wandering away from its designed path are serious when beam power may exceed several kilowatts. At such power ranges, beam power is sufficient to almost immediately liquify the transmission tube, resulting in implosion of the accelerator tube with possible damage to the accelerator. Another undesirable consequences of the beam wandering is the generation of x-rays in a unshielded portion of the transmission tube.
One known solution to this problem is the placement of bending magnets to correct the deflections of the electron beam. However, this is effectual only for static deflections caused by static magnetic fields. Further, the necessity of correcting the beam in addition to focusing and deflecting it, adds complexity to the design of the beam handling magnets.
What is needed is a means for controlling or preventing the deflection of a high energy electron beam produced by an electrostatic accelerator.